Miniaturized superconducting dielectric resonator filters and method of operation thereof

Abstract: An improved design for microwave bandpass cavity filters wherein dielectric resonator elements are mounted in dielectric blocks (there can be more than one resonator element per dielectric block). The dielectric blocks are in turn fitted within the filter housing, and the open ends are covered by shorting plates which are maintained in contact with each resonator surface. The shorting plates may be coated with a superconductive material. The structure results in significant size reduction over prior art filters. In addition, the new design is of higher structural integrity and helps to maintain a consistent output over a wide range of temperatures. By carefully choosing the dielectric materials to insure uniform thermal expansion of the component parts, the filter output is stable over a wide range of temperatures. This allows the filters to be tuned while at cryogenic temperatures, then returned to room temperature for storage or shipping, and finally return to cryogenic temperatures for operating purposes. The filters can be constructed with various configurations and can be operated in single mode, dual-mode, triple-mode, etc.

Description: This invention relates to microwave bandpass filters, and more particularly, to a filter design which allows further substantial miniaturization, and to an improved method of tuning and operation at cryogenic temperatures. The use of dielectric resonators in microwave filters results in a significant reduction in size and mass while maintaining a performance comparable to that of waveguide filters without dielectric resonators. A typical dielectric resonator filter consists of a ceramic resonator disc mounted in a particular way inside a metal cavity. In addition to miniaturization, loss performance, as well as thermal and mechanical stability are also important design objectives for dielectric resonator filters. A number of specific refinements can be incorporated in furtherance of these goals. For instance, in dielectric resonator filters the size of the cavity can be substantially reduced by mounting the dielectric resonator along a base wall of the cavity rather than mounting the resonator in a center of the cavity. This eliminates the need for a centering stem-type mounting, and it allows a reduction in the size of the microwave cavity. See, U.S. Patent No. 4,423,397 issued to Nishikawa, et al. However, it is difficult to attach the dielectric resonator to the base wall in such a way that proper electrical contact is ensured. Conductive glues and the like can result in a change in frequency of the filter, thereby reducing the Q (i.e. quality factor). Moreover, this type of mounting is prone to the thermal expansion caused by wide temperature variations, and to the mechanical vibrations that must be endured when the filter is used in space applications. Multiple mode filters also can provide further miniaturization over single mode filters. For instance, single, dual and triple mode dielectric resonator waveguide filters are known (See U.S. Patent No. 4,142,164 by Nishikawa, et al., issued February 27th, 1979; U.S. Patent No. 4,028,652 by Wakino, et al. issued June 7th, 1977; Paper by Guillon, et al. entitled "Dielectric Resonator Dual-Mode Filters", Electronics Letters, Vol. 16, pages 646 to 647, August 14th, 1980; U.S. Patent No. 4,675,630 by Tang, et al. issued June 23rd, 1987; U.S. Patent No. 4,652,843 by Tang, et al. issued March 24th, 1987; and U.S. Patent No. 5,083,102 by Zaki.). The use of superconductors is a more recent advance which holds good potential. For example, a hybrid dielectric resonator high temperature superconductor filter is known which utilizes a plurality of resonators in a cavity where each resonator is spaced from a conductive wall of the cavity by a superconductive layer. The superconductive layer is capable of superconducting at temperatures as high as about 77° K. Existing superconductive filters cannot produce repeatable results when these filters are tuned at cryogenic temperatures, then allowed to return to room temperature and subsequently return to cryogenic temperatures. As a result, a heat exchanger is necessary to maintain the filter housings at or below the critical temperature of the superconductor after the filters have been tuned. Any further miniaturization gained by the use of superconductors is undermined by the need to employ a bulky heat exchanger or like refrigerant. Finally, U.S. Patent No. 4,881,051 by W.C. Tang, et al. issued November 14th, 1989 describes a dielectric image-resonator multiplexer. The use of image resonators, as disclosed in the Tang '051 patent, allows smaller sectional resonator elements with some degradation in loss performance. It would be greatly advantageous to improve the miniaturization and loss performance of a dielectric resonator filter by incorporating superconductive materials and image resonators in a simplified design, and to improve the thermal and mechanical stability of the filter by using mounting blocks. It is an object of the present invention to provide a dielectric resonator filter that can be used in conventional and cryogenic applications. It is a further object of this invention to provide a dielectric resonator filter that is compact in size with a remarkable loss performance compared to previous filters. It is still a further object of the present invention to provide a dielectric resonator filter in which thermal stability problems associated with operation of previous filters at cryogenic temperatures have been reduced or eliminated. The filter is capable of producing repeatable performance results as temperature changes from cryogenic to room temperature and then back to cryogenic without readjusting the tuning screws. The present invention provides a microwave filter as defined in claim 1 and a method of operation as defined in claim 29. A filter according to the preamble of claim 1 is known from patent document US-A-4630009. The quality factor ("Q") of the support block improves as the ambient temperature changes from 300°K to 77°K. Consequently, the use of the dielectric block to support the resonator element in cryogenic applications considerably reduces the size of the filter without detracting from performance. The dielectric block is sized and shaped relative to the cavity so that the block fits securely within the cavity. The block has an interior that is sized and shaped to hold the dielectric resonator. The support block also remains in contact with a shorting plate that is located within the filter, and the support block preferably holds the shorting plate in a fixed position. As previously described, the role of the shorting plate is to reduce size and improve spurious-free performance. The maximum attainable spurious-free window for C-band dielectric resonator filters is typically 500 MHz to 800 MHz. In contrast, the filter of the present invention has an upper spurious-free window of more than 1.2 GHz. In operation, the microwave cavity resonates in at least one mode at its resonant frequency, there being one tuning screw for each mode and for each resonator within the cavity. There is one coupling screw for every two modes that are coupled within the cavity. The cavity housing has suitable openings to accommodate the tuning screw(s) and coupling screw(s). One of the major shortcomings of existing filters with tuning screws has been their thermal instability across wide temperature ranges. The present invention is stable to ensure performance repeatability as the temperature changes from cryogenic (during tuning and testing) to room temperature (during storage) and then back to cryogenic temperature. The invention also provides a method of using the microwave filter as described above, the method including the steps of tuning the filter while at cryogenic temperatures, raising the temperature of the filter to ambient temperature for storage or transport, and deploying and operating the filter at cryogenic temperatures. Despite the wide temperature variations and thermal expansion/contraction, the filter can produce repeatable results without adjusting the tuning screws after the filter is first tuned at cryogenic temperatures. Other advantages and results of the invention are apparent from the following detailed description by way of example of the invention and from the accompanying drawings. In the drawings: Figure 1 is a schematic side view of a prior art dielectric resonator cavity with a resonator element mounted centrally in the cavity; Figure 2 is a schematic side view of a prior art dielectric resonator cavity with a resonator element mounted flush on a bottom surface of said cavity; Figure 3 is an exploded perspective view of a dielectric resonator filter in accordance with the present invention, said filter having two cavities with one dielectric resonator in each cavity, the two cavities being separated by an iris; Figure 4 is a partially cut-away perspective view of a dielectric block used in the filter shown in Figure 3; Figure 5 is a perspective view of an alternate embodiment of the block of Figure 4; Figure 6 is a perspective view of a shorting plate made of invar with one surface thereof plated with a suitable metal; Figure 7 is a perspective view of a shorting plate made of a dielectric substrate with one surface thereof coated with a suitable metal or high temperature ceramic material; Figure 8 is a graph illustrating the RF performance of a dielectric resonator filter as described in Figure 3 where blocks of said filter are made out of sapphire; Figure 9 is a graph illustrating the RF performance of the dielectric resonator filter of Figure 3 where the blocks of the filter are made of "D4"; Figure 10a is a graph showing the RF performance of the dielectric resonator filter disclosed in Figure 3 before vibrations; Figure 10b is a graph showing the RF performance of the dielectric resonator filter disclosed in Figure 3 after vibrations; Figure 11 is a graph showing the RF performance of a dielectric resonator filter shown in Figure 3 where shorting plates of the filter are made from high temperature superconductive films deposited on a dielectric substrate; Figure 12 is an exploded perspective view of a dielectric resonator filter having two cavities with two dielectric resonators in each cavity; Figure 13 is an exploded perspective view of a dielectric resonator filter having four cavities with one dielectric resonator in each cavity; Figure 14 is an exploded perspective view of a further embodiment of a dielectric resonator filter having four cavities where there are two dielectric resonators located in each cavity; Figure 15 is a graph showing the RF performance of an eight-pole filter having a shorting plate as described in Figure 6; and Figure 16 is a graph showing the RF performance of an eight-pole filter having a shorting plate as described in Figure 7, said filter operating at cryogenic temperatures. Figure 1 shows a dielectric resonator 2 located on a support 4 in a cavity 6. The resonator 2 is supported in a plane z = 0 in which the tangential field of the HEE, TEE or TME modes vanishes. In Figure 2, the same reference numerals as those of Figure 1 are used to describe the same components. However, here the dielectric resonator 2 is mounted on a base 8 of a cavity 10. The base 8 is a conducting wall, and if perfectly conductive it would not change the resonant frequencies of the modes. Hence, the conducting base 8 can be used to reduce the size of the cavity 10 by eliminating the support 4 of Figure 1. Unfortunately, it is difficult to attach the dielectric resonator 2 to the conducting base 8 as glues and the like may damp the resonations, thereby reducing the quality factor Q of the resonator 4. It has also been found that the electrical contact between the dielectric resonator 2 and conducting base 8 is adversely affected by thermal expansion, especially since glues and the like are prone to cracking at cryogenic temperatures. Furthermore, if the conducting plane or base 8 is formed of conventional materials there will inherently be a small resistance. Any amount of resistance will likewise degrade the quality factor Q. It is therefore important to devise a support for the resonator which maximizes the resonator loaded Q while withstanding mechanical vibrations and also meeting all filter thermal requirements. For use of a filter at cryogenic temperatures, the loaded Q of the resonator will be improved by replacing the conducting plate 8 shown in Figure 2 by ceramic materials that become superconducting at liquid nitrogen temperatures. The loss tangent of dielectric resonator materials decreases as the temperature decreases. Therefore, by combining high temperature superconducting materials with dielectric resonators, it is possible to achieve a dielectric resonator filter with superior loss performance for cryogenic applications. Typically, microwave cavity filters have tuning screws that must be tuned at temperatures approximating those in which the filter will ultimately be deployed. Consequently, superconductive filters intended for space applications must be tuned at cryogenic temperatures. However, after they have been tuned the filters must be stored prior to deployment. It would be most convenient to store the filters at room temperature, but the large temperature swing back to room temperature would cause significant thermal expansion. With the prior art superconducting filters, the thermal expansion of component parts is non-uniform, and these filters lose their initial tuning as they warm to ambient temperatures. For this reason, heat exchangers or other temperature control means must be used to maintain the prior art filters at cryogenic temperatures after the filters have been tuned. The unique filter structure of the present invention promotes uniform thermal expansion, thereby eliminating the need for temperature control. The filter structure of the present invention keeps the performance repeatable as the temperature changes from cryogenic to room temperature and then back to cryogenic. An embodiment of the present invention is shown in Figure 3. Here, a dielectric resonator filter 12 has two cavities 14, 16 that are separated by an iris 18 containing an aperture 20. The iris 18 could be in the form of a rectangular slot, a cross-slot or various other known shapes. The illustrated aperture is shown only partially but is a cruciform aperture. The filter 12 has a housing 22 that includes a cover 24 and two end plates 26. The housing 22 can be made of any known metallic materials that are suitable for waveguide housings, for example, invar. Screws to secure the cover 24 and end plates 26 onto the housing 22 are not shown. The filter has an input 28 and output 30, both of which are shown to be exemplary microwave probes that are mounted in holes 32, 34 respectively of the housing 22. Each cavity 14, 16 contains a dielectric block 36, which in turn contains a dielectric resonator 38 and a shorting plate 40 connected thereto. The block 36 is sized and shaped to fit within the cavity in which it is located. The block 36 of the present embodiment is solid except for a recess 42 that corresponds to a size and shape of each resonator 38 and shorting plate 40. Preferably, each block 36 fits within the cavity in which it is located and the resonator 38 and shorting plate 40 in turn are held snugly within the block 36 in a fixed position. The dielectric block 36 may be commercially available TRANS-TECH D-450 series material with a coefficient of thermal expansion (CTE) of 2.4 ppm/°C. However, other materials are also suitable, such as sapphire with a CTE of 8.4 ppm/°C, or quartz single crystal with a CTE of 7.10 ppm/°C parallel to the Z-axis and 13.24 ppm/°C perpendicular to the Z-axis. To keep performance repeatable as outside temperatures change from cryogenic to room temperature and then back to cryogenic, the CTE of the dielectric blocks 36 should substantially match that of the housing 22. This way, these components will expand and contract at substantially the same rate, and this will ensure performance repeatability as the ambient temperature changes from cryogenic to room temperatures (i.e. during shipping and storage) and then back to cryogenic temperatures (during testing and operation). The dielectric resonators may be made of commercially available Murata M series material with a CTE of 7.0 ppm/°C. In some filters, the dielectric blocks 36, the housing 22 and the dielectric resonators 38 will be made of different materials having substantially the same CTE. While it is preferred to have the same CTE between the resonators and the blocks, filters manufactured in accordance with the present invention can have dielectric resonators with a substantially different CTE from the dielectric blocks. The matched CTEs ensure thermal stability across a wide temperature range. During testing, a filter as described in Figure 3 was tuned initially at cryogenic temperature. The filter was then recycled a number of times between cryogenic temperature and room temperature. No performance degradation was observed as the filter was retested at cryogenic temperatures. After the intial tuning (such as during shipping and storage), there is no longer any need to use a heat exchanger or refrigerant to maintain the filter at cryogenic temperatures. The filter of the present invention remains stable despite ambient temperature fluctuations. The shorting plates 40 are preferably coated with a high-conductivity non-oxidizing metal such as gold or a high-temperature superconducting material. The role of the shorting plate 40 is to shift down the resonant frequency of the dielectric resonator element, thereby allowing the use of the smaller resonator. In addition, the flush mounting of the resonator element eliminates the need for the spacer/support 4 of Figure 1, and this too helps to reduce the filter size. Spring washers (e.g., belleville washers) 44 are used to support and hold the dielectric resonators 38 and shorting plates 40 in place inside the support block 36. The spring washers 44 are inserted between the end plates 26 and the shorting plates 40 to urge the shorting plate 40 into good contact with the resonator 38. This way, the spring washers 44 help to provide a firm and constant pressure between the dielectric resonators 38 and the shorting plates 40. The constant pressure insures good electrical contact despite the large amounts of thermal expansion and contraction which may take place. The spring washers 44 may be any type of metal or other material. However, to improve loss performance the spring washers 44 should be plated with a high-conductivity material such as silver, gold or copper. Silver-plated stainless steel spring washers 44 achieve good results. The housing 22 as well as the block 36 contains suitable openings 46 to receive tuning and coupling screws 48, 50. In operation, the filter 12 can be operated in a dual HE mode to realize a four-pole dual-mode response or a TE mode to realize a two-pole single mode filter or a TM mode to realize a two-pole single mode filter. The filter 12 shown in Figure 3 operates in a dual-mode. Energy is coupled into the cavity 14 through input probe 28. Energy is coupled between the two modes within the cavity 14 by coupling screw 50 and is coupled through the aperture 20 into the cavity 16. Energy within the cavity 16 is coupled between the two modes by coupling screw 50 and exits the cavity 16 through the output 30. It can be seen that the blocks 36 are sized and shaped to substantially fill each of the cavities 14, 16. In Figure 4, there is an enlarged perspective view of a block 36 of Figure 3. In this embodiment the hollow portion 42 has a cylindrically-shaped section that is sized to receive the resonator 38 and a square section adjacent thereto that is sized and shaped to receive the shorting plate 40. It can also be seen that when inserted, the resonator 38 and shorting plate 40 (not shown in Figure 4) will fit snugly within the hollowed portion 42. In Figure 5, there is shown a perspective view of another block 52, which can be used as an alternative to the block 36 of Figure 4. The block 52 has an interior 54 that is sized and shaped to receive a cylindrical resonator 38 (not shown in Figure 5) and a shorting plate 40 (not shown in Figure 5). The block 52 has four legs 56 that are identical to one another. Each leg 56 has an arc-shaped interior surface 58. The resonator 36 rests against these arc-shaped surfaces 58 and against a base 60 so that the resonator is snugly supported within the block 52. The shorting plate is supported on shoulders 62 of each of the legs 56. The shorting plate is also supported snugly on the shoulders. The block 56 has openings 46, 64 to receive tuning and coupling screws 48, 50 (not shown in Figure 5). The openings 46 could be blind or through. The outside dimensions of the block 52 are chosen so that the block fits snugly within the cavity. The inside 5 dimensions are chosen so that the resonator and shorting plate fit snugly within the block. In comparison with the block 36, with the block 52 material has been removed to reduce the mass and to improve the loss performance. In Figure 6, there is shown a shorting plate 40 having a surface 66 that contacts the resonator 38 (not shown in Figure 6) when the shorting plate and resonator are installed within a block (not shown). The contact surface 66 is plated with silver or gold in order to reduce the RF losses. In Figure 7, in a further embodiment a shorting plate 68 has a contact surface 70, which is a thin film layer made out of gold or silver deposited on a dielectric substrate 72. The shorting plates 40, 68 shown in Figures 6 and 7 can be used in the filter 12 for cryogenic or conventional room temperature applications. For cryogenic applications, the thin film layer for the contact surface of the shorting plate can be made out of high temperature ceramic materials that become superconductors at cryogenic temperatures (e.g. 77° K or lower) such as yttrium barium copper oxide (YBCO) or thallium barium copper calcium oxide (TBCCO). The dielectric substrate 72 can be made out of lanthium aluminate or sapphire or any other suitable dielectric substrate material. As previously mentioned, the role of the shorting plate 40 is to shift down the resonant frequency of the dielectric resonator as this reduces the filter size. The shorting plates 40 act as image plates, and this is similar in concept to the dielectric image-resonator multiplexer set forth in U.S. Patent No. 4,881,051 issued to W.C. Tang, et al. on November 14th, 1989. However, a true image plate would cover an entire wall of the microwave cavity (for example, as in Figure 2 of the present application), and this in turn allows the resonator 2 to be cut in half. The shorting plates 40 of the present invention cover a significant portion of one wall of the microwave cavity. They can therefore be considered image plates, although not full image plates as described above. Nevertheless, image resonance can be incorporated to varying degrees, and this is true of single and dual-mode filter embodiments. The use of high temperature superconductor materials, instead of gold or silver, significantly improves the loss performance of the dielectric resonator filter for cryogenic applications. It is not necessary that the shorting plate have a square shape. The shorting plate could be rectangular, circular or any other shape or any size so long as it is large enough to cover the circular cross-sectional shape of the dielectric resonators. The dielectric blocks could also be any suitable shape as long as they are sized and shaped to fit snugly within the cavity and have an interior that is sized and shaped to securely support the dielectric resonator and shorting plate. For example, the blocks could have a cylindrical shape and still be used in a square or rectangular-shaped cavity so long as they are sized to fit snugly within the cavity. Further, if the cavity had a cylindrical shape, the blocks could have a square rectangular shape or a cylindrical shape so long as they had a size and shape to fit snugly within the cavity. Figures 8 and 9 illustrate the insertion loss and return loss of a four-pole filter as described in Figure 3 measured at room temperatures. The results in Figure 8 were achieved with the blocks 36 made out of sapphire while those in Figure 9 were achieved with the blocks 36 made out of "D4". The shorting plates 40 used for both Figure 8 and Figure 9 were made out of silver plated invar. Although conventional dielectric resonators can be designed to provide a similar RF performance, they will be considerably larger in size and mass. The size and mass reduction of filters constructed in accordance with the present invention can be more than 50% compared to conventional dielectric resonator filters. When compared to the planar dual-mode filter design described in U.S. Patent No. 4,652,843, size savings of 80% and mass savings of 50% have been achieved. When used in space, the filter must be capable of surviving stringent mechanical vibrations. Figure 10a shows the insertion loss and return loss results of a filter constructed in accordance with Figure 3 before being exposed to typical space-application vibration levels and Figure 10b shows the insertion loss and return loss results after vibration. It can be seen that the results in Figures 10a and 10b are essentially the same and that therefore a filter constructed in accordance with the present invention is capable of withstanding space-application vibration levels. Figure 11 shows the insertion loss and return loss results of a four-pole dual-mode filter constructed in accordance with Figure 3 at cryogenic temperatures. The shorting plate 40 used in the filter was the plate 68 described in Figure 7 with a high temperature superconductor TBCCO thin film layer 70 covering the substrate 72. It can be seen that the filter has a relatively narrow bandwidth (close to 1%) and exhibits a small insertion loss. By comparing the results of Figures 9 and 11, it can be seen that the use of high temperature superconductor materials considerably improves the loss performance of the filter. In Figure 12, there is shown a dielectric resonator filter 74 with two cavities 76, 78 in a housing 80. The same reference numerals are used for those components in Figure 12 that are the same or similar to components of the filter 12 in Figure 3. The housing 80 includes a cover plate 82 and two end plates 84. The cavities 76, 78 are separated by an iris 86 containing one aperture 88. As with the filter 12, the aperture can be any suitable shape, but the illustrated aperture 88 is in the form of a slot. The housing 80, including the cover 82 and end plates 84 can be made of any suitable metal, for example, invar. The cover 82 has two tapped holes 89 for receiving tuning screws (not shown). Each of the cavities 76, 78 contains a dielectric block 90 that has two hollowed portions 42. Each hollowed portion 42 receives a resonator 38 and shorting plate 40. Springs 44 ensure that good contact is maintained between the shorting plate 40 and the adjacent resonator 38. Each block 90 has one hole 91 in a top surface thereof to receive the tuning screw (not shown) that extends through each hole 89 of the cover 82. As with the filter 12, the blocks 90 contain various openings 46 for receiving tuning screws (not shown) and coupling screws (not shown). The tuning screws enter the block 90 at a 90° angle and the coupling screws enter the block 90 at a 45° angle. The filter 74 has an input 28 and an output 30 which are mounted in holes 32, 34 respectively in cavity 78. The input and output are probes. Tiny holes 92 around the periphery of the housing 80 including the cover 82 and end plates 84 are sized to receive screws (not shown) so that the various components can be held together. The tuning and coupling screws, if any, have been omitted from Figure 12 because the number of screws will vary with the number of modes in which the filter is to be operated and the location of the screws is known to those skilled in the art. In operation, the dielectric resonators 38a, 38b, 38c and 38d can operate in the HE mode to realize an eight-pole dual-mode filter or either the TE mode or the TM mode to realize a four-pole single mode filter. The blocks 90 support the resonators 38a, 38b, 38c and 38d in a bottom portion in each of the cavities 76, 78. The hollowed portions 42 are sized and shaped to snugly receive the resonators 38a, 38b, 38c and 38d and the shorting plates 40. Coupling between the dielectric resonators within the same cavity could be controlled by adjusting the spacing between the resonators but is preferably controlled by using tuning screws (not shown) inserted through the cover 82 through tapped holes 89, one hole 89 for each cavity. The holes 89 are aligned with the holes 91 in the blocks 90. The coupling between resonators 38b and 38c of different cavities 76, 78 respectively is achieved through the aperture 88. Energy enters the resonator 38a of cavity 76 and 38b of cavity 76 by the tuning screw (not shown) in the holes 89, 91 of the cavity 76. Energy is coupled from the resonator 38b to the resonator 38c through the aperture 88. Energy is coupled from the resonator 38c to the resonator 38d within the cavity 78 by the tuning screw (not shown) in the holes 89, 91 of the cavity 78. Energy is coupled from the resonator 38d out of the cavity 78 through the output probe 30. In Figure 13, there is shown a dielectric resonator filter 94 having four cavities 96, 98, 100, 102 and four dielectric resonators 38a, 38b, 38c and 38d respectively. Components of the filter 94 that are the same or similar to those of the filter 12 or the filter 74 have been described using the same reference numerals. In general terms, the filter 94 is very similar to the filter 12 except that the filter 94 has four cavities rather than two cavities. The filter 94 has two housings 104, 106 which are virtually identical to one another except for the location of the holes 32, 34 which receive the input and output probes 28, 30 respectively. Each of the housings 104, 106 share common end plates 26 and share a common cover plate 24. The cavities 96, 98 of the housing 104 are separated by an iris 18 containing an aperture 20. The cavities 100, 102 are also separated by an iris 18 (not shown) containing an aperture (not shown). Each of the cavities has a dielectric block 36 with a hollowed portion 42, a shorting plate 40 and a spring 44. The housings 104, 106, the cover 24 and the end plates 26 all have tiny holes 92 around their peripheries so that they can be affixed to one another using screws (not shown). The tuning and coupling screws have been omitted from the drawings for the same reasons as given for Figure 12. In operation, the dielectric resonators 38a, 38b, 38c, 38d can operate either in a HE mode, TE mode or TM mode to achieve either an eight-pole filter or a four-pole filter as previously discussed with respect to filter 74. The embodiment shown in Figure 13 is set up for dual-mode operation because of the presence of openings 46 at a 45° angle to receive coupling screws. Energy is coupled into the cavity 96 through input probe 28 to the dielectric resonator 38a. Energy is coupled between the resonators 38a and 38b through aperture 20 of the iris 18 located in the housing 104. Energy is coupled between the resonator 38b and the resonator 38c through a slot 108 in the cover 24. Energy is coupled from the resonator 38c to the resonator 38d through the aperture 20 located in the housing 106. Energy is coupled from the resonator 38d to the output through output probe 30. The apertures 20 are shown as having a cruciform shape but can have any suitable shape and can be arranged to provide any filter realization such as Chebyshev, elliptic or linear phase functions. Figure 14 shows an eight-pole single mode dielectric resonator filter 110. The filter 110 has eight dielectric resonators 38a, 38b, 38c, 38d, 38e, 38f, 38g, 38h and has the general configuration of two filters 74 as shown in Figure 12 combined together. The same reference numerals have been used for the filter 110 for those components that are the same or similar to the components used in the filter 74. A housing 112 has two cavities 114, 116 that are separated by an iris 118 containing an aperture 120. The housings 112, 122 share a cover plate 124 that contains a slot 126 and share common end plates 84. The housing 122 has an iris 118 with an aperture 120 (not shown in Figure 14), the aperture being located between the resonators 38b and 38c. The tuning and coupling screws have been omitted from the drawing for the same reasons given for Figure 12. The filter 110 can be operated in a single mode or dual mode. When the filter 110 is used as a single mode filter, the openings 46 that extend into the blocks 90 at a 45° angle would be omitted because coupling screws are not required. In operation, energy is coupled into the resonator 38a through the input probe 28. Energy is coupled from the resonator 38a to the resonator 38b by controlling the spacing between the resonators. Energy is coupled from the resonator 38b to the resonator 38c through the aperture 120 (not shown) in the housing 122. Energy is coupled between the resonator 38c and the resonator 38d and is controlled by controlling the spacing between these resonators. Energy is coupled from the resonator 38d through the slot 126 to the resonator 38e. Energy is coupled from the resonator 38e to the resonator 38f through the spacing between these two resonators. Energy is coupled from the resonator 38f through the aperture 120 of the housing 112 through the resonator 38g. Energy is coupled from the resonator 38g to the resonator 38h by controlling the spacing between these resonators. Energy is coupled from the resonator 38h out of the filter through the output probe 30. The coupling between adjacent resonators within the same block 90 can, alternatively, be controlled using tuning screws (not shown). Figure 15 shows the measured performance of an eight-pole filter constructed in accordance with the filter 94 shown in Figure 13. The filter was constructed using the shorting plate shown in Figure 6. In Figure 16, the same filter 94 was used except that the shorting plate shown in Figure 7 was substituted for the shorting plate shown in Figure 6 and the filter was operated at cryogenic temperatures. By comparing Figures 15 and 16, it can be seen that the insertion loss performance of the filter 94 is considerably improved when the filter is operated at cryogenic temperatures using high temperature superconductor materials for the shorting plates 40. The results shown in the graphs of this application are examples only. While various configurations of filters are shown in the drawings, it will be readily apparent to those skilled in the art that other configurations could be utilized as well within the scope of the attached claims. For example, a filter could have three dielectric resonators and could be a three-pole or a six-pole filter, or a filter could have five, six or seven resonators or more than eight resonators. The filter can be operated in either a single mode or a dual mode. A filter can be operated at ambient temperatures or, by using shorting plates having a thin film of high temperature superconductor film thereon, the filter can be operated at cryogenic temperatures. In accordance with the above-described structure, it becomes possible to use a filter by tuning it at cryogenic temperatures (approximating those in which the filter will ultimately be deployed), and then storing the filter at room temperature prior to deployment. This is most convenient for satellite applications since the filters can be tuned by the manufacturer well before the filters are to become operational. The thermal expansion of component parts is uniform, and the filter does not lose its initial tuning as it warms to ambient temperatures. The present invention also encompasses the above-described method of using a filter by: 1) tuning at cryogenic temperature; 2) storing at room temperature; and 3) deploying at cryogenic temperature (in space). Various changes in the structure of the filter or method of its use, within the scope of the attached claims, will be readily apparent to those skilled in the art. For example, the cavities could have a cylindrical shape with the blocks remaining square or rectangular or the blocks could have a cylindrical shape with square, rectangular or cylindrical cavities. Various shapes will be suitable for the blocks. Having now fully set forth a detailed example and certain modifications incorporating the concept underlying the present invention, various other modifications will obviously occur to those skilled in the art upon becoming familiar with the underlying concept. For instance, although the present invention is especially suited for cryogenic applications, it should be understood that the filter of the present invention is equally well-suited for conventional use at room temperature. A smaller size and better loss performance will still be attained.

Claims: A microwave filter having in combination: (a) at least one cavity (14) having at least one dielectric resonator (38) therein; (b) said cavity resonating in at least one mode at the resonant frequency of said cavity, there being a respective tuning screw (48) for each mode and for each resonator within said cavity, and one coupling screw (50) for every two modes that are coupled within said cavity; (c) said filter having input (28) an output (30) and operatively connected thereto; (d) said filter being characterized by at least one dielectric block (36) disposed in said at least one cavity, with said at least one dielectric resonator connected thereto and being located within said block, said at least one resonator having an associated shorting plate (40) connected thereto said shorting plate being located within said block, said block having a size and shape relative to said cavity so that said block fits tightly within said cavity, said block having an interior with a size and shape to hold said dielectric resonator and said shorting plate within said block in a fixed position, said block having suitable openings to accommodate said tuning and coupling screws. A filter as claimed in Claim 1 wherein the respective block (36) has at least three areas of contact with said corresponding cavity. A filter as claimed in Claim 2 wherein there are at least two dielectric resonators (38) contained separately within one block with each resonator having an associated shorting plate (40). A filter as claimed in any one of Claims 1, 2 or 3 wherein the respective block (36) has a size and shape to substantially fill said corresponding cavity. A filter as claimed in any one of Claims 1, 2 or 3 wherein the respective block (36) has a size and shape to substantially fill said corresponding cavity (14), said respective block being solid except for a hollowed portion (42) that corresponds to a size and shape of each resonator (38) and associated shorting plate (40) contained therein and except for openings to accommodate said tuning screws (48) and any coupling screws (50). A filter as claimed in any one of Claims 1, 2 or 3 wherein there are at least two cavities (14,16)and at least one of the cavities has a rectangular shape and the respective block (36) has a similar rectangular shape corresponding to an interior of said corresponding cavity. A filter as claimed in Claim 1 wherein each dielectric resonator (38) is held in contact with an associated shorting plate (40) using a corresponding spring. A filter as claimed in Claim 7 wherein the associated shorting plate (40) is comprised of a metallic material. A filter as claimed in Claim 8 wherein the associated shorting plate (40) is comprised of high temperature superconductive thin films deposited on a dielectric substrate. A filter as claimed in Claim 8 wherein the associated shorting plate (40) is comprised of high temperature superconductive thin film deposited on a dielectric substrate. A filter as claimed in Claim 1 wherein the dielectric resonators (38) operate in a single mode that is selected from the group of a TEE mode or a TME mode. A filter as claimed in Claim 1 wherein the dielectric resonators (38) operate in a dual HEE mode. A filter as claimed in any one of Claims 3, 7 or 9 wherein there are two cavities (76,78) with one block (90) in each cavity, each block containing two dielectric resonators (38) and corresponding shorting plates(40), the dielectric resonators being operated in a mode selected from the group of an HEE mode to realize an eight-pole dual-mode filter, a TEE mode to realize a four-pole single mode filter and a TME mode to realize a four-pole single mode filter, there being sufficient tuning screws and coupling screws as required, with means to control coupling between the resonators located within the same block and an iris (86) containing an aperture (88) located between said cavities to control coupling between resonators in different blocks, said blocks containing channels (46) to receive said tuning and coupling screws. A filter as claimed in any one of Claims 1 or 2 wherein there are four cavities (96,98,100,102), with one block and a respective dielectric resonator (38a, 38b, 38c,38d) and associated shorting plate (40) mounted in each block, there being two irises, each iris having two sides, one iris (86) being located between two of said cavities and another iris being located between said other cavity, each iris (86) having an aperture (88) with a shape to permit coupling between the dielectric resonators located on either side of said iris, the filter (94) being operated in a mode selected from the group of an HEE mode to realize an eight-pole dual-mode filter, a TEE mode to realize a four-pole single mode filter, and a TME mode to realize a four-pole single mode filter. A filter as claimed in any one of Claims 1, 2 or 3 wherein there are two blocks (36) with two dielectric resonators (38) mounted in one block and three dielectric resonators (38) mounted in another block, an iris located between said blocks, the coupling between resonators in adjacent blocks being controlled by an aperture (88) located in the iris (86) with means to control the coupling between resonators located in the same block. A filter as claimed in any one of Claims 1, 2 or 3 wherein there are two cavities, with each cavity (76, 78) containing two dielectric resonators (38) and associated shorting plates (40) contained in the respective block of each cavity. A filter as claimed in any one of Claims 1, 2 or 8 wherein there are four cavities (96, 98, 100, 102) with one dielectric resonator (38) and associated shorting plate (40) in each respective block, with one block being located in each cavity. A filter as claimed in any one of Claims 1, 2 or 3 wherein there are four cavities with two dielectric resonators (38a, 38b) and associated shorting plates (40) being located in a respective block in each cavity, there being one block in each cavity. A filter as claimed in any one of Claims 1, 2 or 3 wherein the blocks (90) are comprised of ceramic materials of low loss tangent selected from the group of D4 and sapphire. A filter as claimed in any one of Claims 1, 2 or 3 wherein the associated shorting plate (68) has a dielectric substrate (72), said associated shorting plate having a contact surface (70) that is in contact with a dielectric resonator, said surface having a plating of a material selected from the group of silver, gold and high temperature ceramic materials. A filter as claimed in any one of Claims 1, 2 or 3 wherein the associated shorting plate has a surface (66) that contacts said dielectric resonator, said surface having a coating of a thin film layer of material selected from the group of yttrium barium copper oxide and thallium barium copper calcium oxide. A filter as claimed in any one of Claims 1, 2 or 3 wherein the associated shorting plate is comprised of a dielectric substrate (72) selected from the group of lanthium aluminate and sapphire. A filter as claimed in any one of Claims 1, 2 or 3 wherein the dielectric resonator (38) has a cylindrical shape and the associated shorting plate (40) has a cross-sectional size sufficient to cover a cross-sectional area of said resonator. A filter as claimed in any one of Claims 1, 2 or 3 wherein the associated shorting plate (40) has a surface adjacent the dielectric resonator (38)having a coating of a high temperature superconductive film so that said filter can be operated at cryogenic temperatures. A filter as claimed in Claim 7 wherein the spring (44) for each associated shorting plate (40) is located between a cavity wall and the associated shorting plate to urge said associated shorting plate towards said respective resonator. A filter is claimed in Claim 2 wherein said block and said housing have different coefficients of thermal expansion from said resonator. A filter is claimed in Claim 2 wherein said block, said housing and said resonator are formed of materials having substantially equal coefficients of thermal expansion. A filter as claimed in any one of Claims 1, 2 or 3 wherein the respective blocks are comprised of ceramic materials. A method of operating a microwave filter having at least one cavity (14) with a dielectric block (36) disposed therein, with at least one dielectric resonator (38) and associated shorting plate (40) connected thereto, said dielectric resonator and associated shorting plate being located within said block, said block being sized and shaped relative to said cavity so that said block fits tightly within said cavity, said block having an interior that is sized and shaped to hold said dielectric resonator and said associated shorting plate within said block in a fixed position, said cavity resonating in at least one mode at its resonant frequency, there being sufficient tuning and coupling screws, said filter having an input (28) and output (30), said block being made out of ceramic materials of low loss tangent, said associated shorting plate having a surface (66) adjacent said resonator that is plated with high temperature ceramic materials that become superconductive at cryogenic temperatures, said method being characterized by lowering the temperature of said associated shorting plate to cryogenic temperatures, tuning said filter, raising the temperature of said associated shorting plate to room temperature, lowering the temperature of said shorting plate to cryogenic temperatures on operating said filter at cryogenic temperatures. A method as claimed in Claim 29 wherein the filter is operated to realize a result selected from the group of Chebyshev, elliptic and linear phase functions.

Inventors: DOKAS VAN, MANSOUR RAAFAT R, DOKAS, VAN, MANSOUR, RAAFAT R.
Applicants: COM DEV LTD, COM DEV LTD.
Application year: 1994
Main classification: H01P1
IPC codes: H01P1